The present application relates to a lithium ion secondary battery including a cathode and an anode opposed to each other with a separator in between, an electronic device using the same, an electric power tool using the same, an electrical vehicle using the same, and an electric power storage system using the same.
In recent years, electronic devices represented by a mobile phone and a personal digital assistant (PDA) have been widely used, and it has been strongly demanded to further reduce their size and weight and to achieve their long life. Accordingly, as a power source for the electronic devices, a battery, in particular, a small and light-weight secondary battery capable of providing a high energy density has been developed. In recent years, it has been considered to apply such a secondary battery not only to the foregoing electronic devices but also to various applications represented by an electric power tool such as an electrical drill, an electrical vehicle such as an electrical automobile, and an electric power storage system such as a home electrical power server.
As the secondary battery, secondary batteries using various charge and discharge principles have been widely proposed. Specially, a lithium ion secondary battery using insertion and extraction of lithium ions is considered promising, since the lithium ion secondary battery provides a higher energy density than lead batteries, nickel cadmium batteries, and the like.
The secondary battery includes a cathode, an anode, and an electrolytic solution. The cathode and the anode respectively contain a cathode active material and an anode active material that insert and extract lithium ions. In the secondary battery, in order to obtain a high battery capacity, a lithium composite oxide such as LiCoO2 is used as the cathode active material, and a carbon material such as graphite is used as the anode active material.
In the secondary battery, in general, its operating voltage is from 2.5 V to 4.2 V both inclusive. One of the reasons the operating voltage is allowed to be increased up to 4.2 V even in a single battery is as follows. That is, since the cathode is separated from the anode by a separator, the secondary battery is electrochemically stable.
In terms of performance of the secondary battery, it is demanded to further increase its battery capacity. Meanwhile, in the existing secondary batteries, only about 60% of the theoretical capacity of the lithium composite oxide as the cathode active material is used. Therefore, what we call a remaining capacity exists. Therefore, to use the remaining capacity, it has been proposed to realize a high energy density by increasing the charge voltage to a value larger than 4.2 V (for example, see WO03/019713).
However, in the case where the charge voltage is increased to a value larger than 4.2 V, a large amount of lithium ions is extracted from the lithium composite oxide, and therefore the cathode becomes unstable thermally and electrically. Thereby, side reactions such as a decomposition reaction of the electrolytic solution easily occur, and gas is easily generated in the battery resulting from the side reactions. Therefore, cycle characteristics are lowered, and safety is lowered resulting from battery swollenness.
Therefore, it has been proposed to suppress side reactions even if a charge voltage is increased by adding an aromatic compound to an electrolytic solution (for example, see Japanese Unexamined Patent Application Publication Nos. 07-302614 and 2000-058117). As the aromatic compound, an anisole derivative having π electron orbit in which the reversible oxidation-reduction electric potential is more noble than the cathode electric potential at full charge, an ether derivative having a specific chemical structure, and the like are used. The aromatic compound is reacted in the oxidant atmosphere in the vicinity of the surface of the cathode at a late stage of charging, and suppresses gas generation.
In addition, to improve ion conductivity of lithium ions, it is proposed to add an organic silicon compound having a polysilsesquioxane skeleton to an electrolytic solution or a cathode (for example, see Japanese Unexamined Patent Application Publication Nos. 2005-002159, 2008-171813, and 2003-306549).